1. Field of the Invention
This invention relates to the field of power control in mobile communications networks. More particularly, but not exclusively, the invention relates to control of the uplink transmission power.
2. Description of the Related Art
A user equipment (UE) for the use in cellular communications networks such as the Universal Mobile Telecommunications System (UMTS) has a limited amount of power resources available for transmitting user and control data in the uplink direction to the base stations of the network. Also, the network may limit the UE power allowed to transmit in the uplink direction in order to ensure that the signal achieves a predetermined signal-to-noise ratio.
The allowed or available uplink transmission power may be shared by a number of channels transmitted at the same time. In macro cell coverage scenarios, under deep fades or at the edge of the cell, the UE may often have to operate at or near maximum transmission power.
In case the UE detects that it may have a power problem such as it is required to serve multiple channels with a total transmission power exceeding the available or allowed uplink transmission power, there are two mechanisms foreseen in UMTS to handle such power “shortages”.
The first mechanism is the so-called long-term behaviour. By applying the long-term mechanism, the UE is controlling the data rate used for uplink transmission power.
In UMTS, the network allocates a range of suitable bit rates or transport formats to the UE. The UE selects an appropriate transport format from the allowed set according to its buffer occupancy and power availability.
If the UE is running low on power, then the UE will reduce its data rate by selecting a lower data rate transport format. For example, if a UE is running low on power at the edge of a cell, it will eliminate certain allowed Transport Format Combinations (TFCs) from the set of allowed TFCs given by the network. In this manner, the UE will try to avoid a power problem by selecting an appropriate transport format corresponding to a lower data rate at the beginning of the next transmission frame.
The long term mechanism used in UMTS is described in more detail in the 3rd Generation Partnership Project (3GPP) Specifications, see section 11.4 of the 3GPP TS 25.321, “Technical Specification Group Radio Access Network; Medium Access Control (MAC) protocol specification” and section 6.4 of 3GPP TS 25.133, “Technical Specification Group Radio Access Network; Requirements for Support of Radio Resource Management (FDD)”.
The second mechanism is the so-called short-term behaviour. The UE will apply the short-term behaviour when the UE is already experiencing a power problem such as described above. In this mechanism the uplink transmission power is scaled down such that the maximal transmission power is not exceeded. This mechanism can be applied directly for transmission in the next slot rather than at the beginning of the next transmission frame as in the case of the above described long term behaviour.
The introduction of multiple parallel services such as those available in UMTS has made things even more challenging. In Release 5 of the 3GPP specifications, a High Speed Downlink Packet Access (HSDPA) feature is introduced which supports high data rate transmission in downlink direction. See the 3GPP specification TS 25.308, “High Speed Downlink Packet Access (HSDPA), Overall Description” for more details. However, this service requires transmission of signalling data in the uplink direction using a new physical channel, the so-called High Speed-Dedicated Physical Control Channel, HS-DPCCH. The impact of this could be a significant addition of power strain on the UE. The power requirement for transmission on the HS-DPCCH could be as high as 20-30% of the total available power. This means that HSDPA capable UE's will have higher probability of experiencing a power problem.